Both にしては and わりには allow us to make statements about things in an objective manner.

彼は年の割に若く見えます。
He looks young for his age.
彼にしてはテストの点数が悪かった。
The test score was poor for him.

The difference between the two phrases is subtle but one that means にしては cannot be used with adjectives. First compare these example sentences:

安い割には、質がいい。
The quality is good given that it’s cheap.
安いにしては、質がいい。(✘)
The quality is good given that it’s cheap.

The difference is that にしては requires a mutual understanding between the speaker and listener which adjectives by their very nature cannot provide. “Cheap” is subjective—what is considered cheap by you might not be to me. 100 yen, however, is simple a fixed value. Consequently, both structures are acceptable in the below examples.

100円にしては、質がいい。
For 100 yen the quality is good.
100円の割には、質がいい。
For 100 yen the quality is good.

In terms of nuance, にしては emphasises a feeling of surprise on behalf of the speaker at the outcome whereas わりには is more neutral.

お相撲さんにしては、体が小さい。
He’s pretty small for a sumo wrestler.

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